US 2006/0197990 A1 discloses a superposition of two tally images, thus revealing a hidden image. The hidden image cannot be reconstructed from a single tally image.
WO 97/47487 describes a security device having two simple patterns printed on opposite sides of a substrate, which generate different images when seen in reflection and transmission.
EP 1580025 describes a security device which shows, under a certain viewing angle, optically recognizable features.
WO 2009/010714 discloses a security device comprising a substrate having a viewing region and a first pattern of a first colour and a second pattern of a second colour on a first side of the viewing region. On the second side the first and the second pattern are arranged in register with respect to the first side, where the colours are reversed.